Iron is an essential element for most life on Earth, including human beings. The control of this necessary but potentially toxic substance is an important part of many aspects of human health and disease. Hematologists have been especially interested in the system of iron metabolism because iron is essential to red blood cells. In fact, most of the human body's iron is contained in red blood cells' hemoglobin, and iron-deficiency is the most common cause of anemia.
Understanding this system is also important for understanding diseases of iron overload.
Importance of iron regulation
Iron is essential to life, because of its unique ability to serve as both an electron donor and acceptor.
But iron can also be potentially toxic. Iron's ability to donate and accept electrons means that if iron is free within the cell, it can catalyze the conversion of hydrogen peroxide into free radicals. And free radicals can cause damage to a wide variety of cellular structures, and ultimately kill the cell. To prevent that kind of damage, all life forms that use iron bind the iron atoms to proteins. That allows the cells to use the benefits of iron, but also limit its ability to do harm.
The most important group of iron-binding proteins are the heme molecules, all of which contain iron at their centers. Humans and most bacteria use variants of heme to carry out redox reactions and electron transport processes. These reactions and processes are required for oxidative phosphorylation. That process is the principal source of energy for human cells; without it, our cells would die.
Humans also use iron in the hemoglobin of red blood cells, in order to transport oxygen from the lungs to the tissues and to export carbon dioxide back to the lungs. And iron is an essential component of myoglobin to store oxygen in muscle cells.
Excessive iron is toxic to humans, because excess ferrous iron reacts with peroxides in the body, producing free radicals. Iron becomes toxic when it exceeds the amount of transferrin needed to bind free iron. In excess, uncontrollable quantities of free radicals are produced.
Iron uptake is tightly regulated by the human body, which has no physiologic means of excreting iron and regulates iron solely by regulating uptake. However, too much ingested iron can damage the cells of the gastrointestinal tract directly, and may enter the bloodstream by damaging the cells that would otherwise regulate its entry. Once there, it causes damage to cells in the heart, liver and elsewhere. This can cause serious problems, including the potential of death from overdose, and long-term organ damage in survivors.
Up to 22% of the iron in meat is absorbed, while only 1-8% is absorbed from plant foods. Plant iron tends to be ‘bound’ to other nutrients in food and needs to be broken down in the body before it can be absorbed. This not only slows down the process of absorption but enables the body to limit its overall intake. As a consequence, stores of non-haem iron are low in comparison to haem iron as the body takes only what it needs, absorption decreasing as iron stores increase
Iron absorption can also be reduced by tannins (e.g. in tea) and phytates (found in nuts, grain and seeds). At this point one tends to wonder whether the rumours of vegans suffering from anaemia have substance, research has shown that iron deficiency in vegans is no more common than in the rest of the population.
The absorption of iron from plant foods is improved by the presence in a meal of vitamin C (ascorbic acid), other organic acids such as malic acid (e.g. in pumpkins, plums and apples) and citric acid (in citrus fruits). Laboratory research in which experimental meals were given to 299 volunteers has shown that the inclusion of foods (such as fresh salad, orange juice or cauliflower) providing 70-105mg of vitamin C in each meal increased the absorption of iron. A particularly pronounced effect was seen when 4.5oz cauliflower containing 60mg of vitamin C was added to vegetarian meals, causing more than three-fold increase in iron absorption.
Recommended Adequate Intake by the IOM for Iron:
Code: Select all
Age Iron (mg/day)
0 to 6 months 0.27
7 - 12 months 11
1 to 3 years 7
4 to 8 years 10
9 to 13 years 8
14 to 18 years 11
19 to 50 years 8
51+ years 8
9 to 13 years 8
14 to 18 years 15
19 to 50 years 18
51+ years 8
Low Iron Stores: Not Necessarily Unhealthy
Anemia is a possible downside to lower iron absorption, but there are a few potential upsides:
1. Low iron stores are associated with higher glucose tolerance and therefore could prevent diabetes.
2. High iron stores have been linked to heart disease. Based on an early study, this was believed to be a strong link for a number of years. Now that more evidence has come in, the link appears to be only in cases of very high iron storage levels, such as greater than 200 mcg/l (vegans' ferritin levels are rarely above 100 mcg/l). For now, the Food and Nutrition Board of the Institute of Medicine says, "This body of evidence does not provide convincing support for a causal relationship between the level of dietary iron intake and the risk for coronary heart disease."
3. High iron stores have been linked to cancer.
Human Iron metabolism on wikipedia
Iron on veganhealth.org
Iron on NIH